Is Your Stormwater Outfall Adequate?

April 2, 2018

About the author: Joseph W. McClellan is vice president for the William H. Gordon, Associates, Inc. He can be reached at 703/580-0409 or by e-mail at [email protected].


The landscape in Fairfax County, Va., is continually changing as development progresses. Due to a lack of available vacant land, development projects are much smaller than in times past.

Typically, the smaller projects are defined as “in-fill” and are constrained by the existing developments that surround them. These landmasses are not able to support the area needed to provide adequate stormwater management and BMP facilities in extended detention ponds. Even the smallest pond requires an embankment top width of 12 ft and all of the same access requirements of larger facilities.

There are many instances where the downstream storm drainage system was designed for a development needing much less impervious cover than the current proposed use. The Fairfax County Public Facilities Manual (PFM) states, “an adequate channel shall be defined as a natural or man-made channel or pipe which is capable of conveying the runoff from a two-year storm without overtopping its banks or eroding after development of the site in question, or without causing the flooding of structures from the 100-year storm event.”

Other provisions of the PFM require any piped system to convey the 10-year storm event. A recent “Letter to the Industry” (see further describes information that is being required on every plan.

Analysis of outfall

Currently, Fairfax County staff is requiring the analysis of the outfall be extended to the nearest open channel and include three to five downstream cross sections of the existing conditions. The Fairfax County staff feels they can require this analysis up to the point at which the project is 1% of the overall drainage system—thus, a five acre development could require a 500-acre drainage analysis.

In any instance where the channel capacity has been exceeded or the velocity of the discharge exceeds an allowable erosive velocity—typically 31/2 ft per second—the Fairfax County staff is interpreting the situation as an inadequate outfall. Once drainage exceeds 50-100 acres, it is going to be virtually impossible to contain the flow within the channel and not exceed the allowable velocity. The cost of improvements to rectify the perceived problem and the extent of any new easements offsite would be impossible to determine without a significant engineering study.

These requirements have made it difficult, at best, to define the limits of the investigation necessary to demonstrate an adequate outfall from any site.

William H. Gordon, Associates, Inc. is currently working with the Fairfax County staff to establish specific parameters for determining an “adequate outfall” such that any costs or easement acquisition is reasonable.

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